90D and Canon 300mm f4 L IS USM inconsistencies in exposure

[Wandalynn]

I wanted to see if (1) anyone else has had this problem and (2) if there are any fixes. First, let me say that my Canon 300mm lens has worked perfectly on my 70D, my 6Dii, and my R cameras. Add to that that all of my other Canon lenses perform perfectly with the 90D. I almost exclusively shoot in AV mode, mostly nature closeups, and mostly use evaluative metering, sometimes spot.  Taking pictures with the 90D + 300 through the VF, almost all of them are severely underexposed, almost black. Taking pictures with same lens using Live View, the exposure is fine. Again, that never happens with my other lenses on this camera, and I have no problems with this lens on my other cameras. Input appreciated.

[Waddizzle]

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@Wandalynn wrote:

I wanted to see if (1) anyone else has had this problem and (2) if there are any fixes. First, let me say that my Canon 300mm lens has worked perfectly on my 70D, my 6Dii, and my R cameras. Add to that that all of my other Canon lenses perform perfectly with the 90D. I almost exclusively shoot in AV mode, mostly nature closeups, and mostly use evaluative metering, sometimes spot.  Taking pictures with the 90D + 300 through the VF, almost all of them are severely underexposed, almost black. Taking pictures with same lens using Live View, the exposure is fine. Again, that never happens with my other lenses on this camera, and I have no problems with this lens on my other cameras. Input appreciated.

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I agree with Ernie. The friction created by the act of mounting or dismounting a lens is sufficient to keep lens contacts clean.  If the contacts are not discolored, then there is probably nothing wrong with them.  Just make sure the have spring action.

If you are shooting in Av or Tv mode, then do not use ISO Auto. As a general rule, you do not want give the camera control over two legs of the Exposure Triangle.  Limit the camera to controlling just one leg, or none of them.

Resetting the camera is good idea.  You should be able to see the exposure metering in the viewfinder, even if it is under exposed.  Make sure you do not have Exposure Compensation dialed in.  Make sure your thumb is not accidentally locking exposure as you carry and hold the camera.

I do not advise using Spot Metering handheld.  Tripod, yes.  Handheld, inadvisable.  Spot metering is a little too exact at times.  Be aware that Spot Metering only [meters] with the Center AF point!  I have used Center Weighted Averaging in conjunction with the Center AF point in One Shot mode with pretty good success at nailing the exposure of the subject.

Perhaps there is some quirk with the EF 300mm f/4L IS USM and the 90D.  Yours is the first complaint that I have heard.

[ebiggs1]

"If you are shooting in Av or Tv mode, then do not use ISO Auto. As a general rule, you do not want give the camera control over two legs of the Exposure Triangle."

Spot on!  I am not sure how the camera decides which auto function has top priority but it is a situation to avoid.

If you KNOW your settings it is probably better than letting the camera pick one for you. However most current DSLRs are very good at it.

[Wandalynn]

Thank you for your suggestions and the info on spot metering. I am guilty of using Auto ISO with AV but also frequently set the ISO.